A common goal for commercial wind-turbine manufacturers is to design and produce a wind-turbine that provides the lowest possible cost of energy (COE) throughout the operational life of a wind-turbine. The COE is determined by a comparison of total yearly costs to yearly energy produced. Thus, the COE is minimized by lowering turbine cost while simultaneously increasing the yearly energy capture.
At present, essentially all commercial wind-turbines have a two or three-bladed rotor rotating about a horizontal axis. The rotor is composed of a central rotor hub and the blades, which define a blade-root diameter located at a junction between the central rotor hub and each blade. Each blade is rigidly attached to the central rotor hub with a blade bearing, which prevents movement of the blade relative to the central rotor hub in all directions except rotationally along the blade's span direction. The rotational degree of freedom is used to pitch the blade into or away from the wind, thereby regulating mechanical power produced.
The blade bearing is also supplemented with a pitch system that includes mechanical actuators and gears, a bearing lubrication system, a slip-ring to pass power to the mechanical actuators, and a back-up power supply. The back-up power supply allows pitch control during emergency power outages.
An optimum blade is generally denoted as having a blade shape with a required blade strength and a minimum total production cost, subject to constraints on maximum chord, but not on blade-root diameter.
Due to rising costs of the rotor and pitch system that use traditional blades, the blade-root diameter has been limited to sizes below optimum values determined solely on blade structural needs. Consequently, blades are heavier and costlier than the optimum blade. At large rotor diameters, the limited, non-optimal, blade-root diameter results in high edge-wise loads that limit the length of the blade and, hence, the annual energy capture.
Wind-turbine designs employing a single rotor bearing are described in U.S. Pat. No. 6,285,090; WO 02/057624; U.S. Pat. No. 6,872,049; WO 01/21956; and DE 29609794. Each of these designs uses a three-bladed rotor with blade pitch bearings, and hence inherit the deficiencies of a non-optimal blade described above. Consequently, advantages professed by the prior art are limited to relatively small changes to supporting structures and improved service access to a rotor interior.
A Gamma wind-turbine, manufactured by West Energy Systems, Taranto, Italy, differentiates itself from other designs by varying nacelle yaw angle to control the mechanical power produced by the rotor. In this design, the blades are directly fixed to the central rotor hub, which avoids having to use blade bearings. The central rotor hub is supported by two teeter hinges, which are themselves attached to a conventional shaft having a small diameter and bed-plate structure. Spacing between the two teeter hinges is necessarily small due to the small diameter of the shaft. In this configuration, small spacing between the teeter hinges replaces the blade bearing diameter as the factor limiting the hub dimension, and, thereby, the blade-root diameter. This also results in the blades being heavier and costlier than the optimum blade.
The net result of all current designs is an increase in total capital for turbine costs, which rises much faster due to rotor diameter limitations than the annual energy capture. Consequently, lowering the COE is a difficult, and sometimes impossible task.
Thus, there is a need for a wind-turbine design that simultaneously lowers the initial capital cost of the wind-turbine while simultaneously increasing the annual energy capture with respect to current designs. Accordingly, one example objective for the present invention is to provide a wind-turbine using optimal blades to maximize energy capture for a given blade cost.
Another example objective is to provide a wind-turbine rotor and drivetrain configuration in which the aerodynamic and gravity loads are carried through an external skin to reduce the amount of material employed, which in turn reduces turbine cost.
Another example objective is to provide a wind-turbine that uses a drivetrain with fewer parts than traditional configurations, which would also reduce turbine cost.
Finally, another example objective is directed to selection of a direct-drive generator. When this is selected, it is an objective to provide a drivetrain with multiple function capability. The multiple function capability can include for example, (a) load carrying, (b) back-iron for a generator, and (c) external enclosure for weather protection and generator-heat dissipation.